1. Field of the Invention
The present invention relates to electrical power systems and, in particular, concerns a power generation system that generates electrical power from solar energy through the use of cascaded thermal electric converters and generators.
2. Description of the Related Art
Power generation in industrialized countries at the present time often requires the use of environmentally unfriendly systems. In particular, the increase use of fossil fuel based power plants in the generation of electrical power has resulted in significant air pollution problems throughout both the industrialized and developing world. Alternatives to fossil fuel plants at the moment appear to continue to have significant environmental problems or are impractical for large scale implementation. For example, the use of hydroelectric power generation facilities often results in the interruption of migration routes for fish and suitable locations for hydroelectric facilities are often limited. The use of nuclear power plants is also an alternative to fossil fuel based power plants, however, nuclear power plants typically produce radioactive waste which presents a whole host of additional environmental problems.
These environmental problems have led to the development of various types of solar power systems, which create electricity from solar radiation. The typical solar cell that is in use today is generally a semiconductor solar cell that converts light energy from the sun into electric potential energy. In particular, photons interact with a depleted region of a p-n junction and produce electron-hole pairs therein. The electrons freed on the n side of the p-n junction recombine with the holes created on the p side, thus, resulting in an increased electric potential across the p-n junction. However, one difficulty with existing solar cells is that they are very expensive to manufacture, are relatively inefficient and require a large area of exposure to sunlight to generate significant electrical power.
Another type of solar base power system comprises systems where thermal energy from the sun is concentrated so as to heat a particular liquid which can then be used for heating purposes or can be used to drive a turbine to generate electrical energy. Again, the efficiency of these types of systems is relatively low and cost prohibitive to allow for wide spread use as a power generation system.
In particular applications, most notably space applications, different types of solar based power systems have been developed. These systems include thermal electric converters that directly convert heat energy into electrical energy. One such system is a thermionic electrical converter (referred to as a TEC) that receives an input flow of thermal energy at a first end, having a high temperature T.sub.H, converts a portion of the input thermal energy directly into electrical energy, and expends the unconverted thermal energy through a second end having a lower temperature T.sub.L. In general, a thermionic converter operates between a particularly high temperature difference such that there is ionization of the material comprising the TEC which results in current flow. One example of a thermionic converter is a Cesium-Barium thermionic converter (Cs--Ba) which will produce electrical energy when exposed to relatively high temperature differences, e.g. on the order of 1500 to 2000 K. These types of thermionic converters are described in greater detail in an article entitled: Terrestrial Solar Power System Based on CS--BA Thermionic Converter published in Space Technology and Application International Forum 1999 by A. Ya Ender et al.
Since TEC's expend a substantial portion of the input flow of thermal energy without converting it into electrical energy, TEC's are relatively inefficient. To address this problem, various bimodal thermoelectric generators which incorporate both thermionic converters (TEC's) and a lower temperature alkaline metal thermal to electric converter (AMTEC) has been proposed. The efficiency of a combination TEC and AMTEC device is improved as the AMTEC device produces additional electrical power by using the unconverted thermal energy that is expended by the TEC.
While these particular systems improve the efficiency of transferring solar heat into electrical energy, these systems still are relatively inefficient such that the ratio of the cost of the device over the electrical energy produced is still too high for wide spread use for earth based power systems. A further difficulty that all of these solar power systems have is that the time period of operation is typically limited to only that portion of the day in which the system will receive direct sunlight. It will be appreciated that this time period may be relatively short due to the sun setting at night and also due to the fact that the sun may be located so low on the horizon during the early morning and late afternoon hours such that there is insufficient sunlight to produce heat energy with the temperatures necessary to operate thermal electric converters such as TEC's and AMTEC's. In fact, it is generally understood that for earth based solar power systems, the amount of time the sun is sufficiently directly incident on the power system to produce electrical power is limited to only 5 hours per day. During the remaining 19 hour period, the sun is either set or is low enough on the horizon that it is not providing sufficient solar flux to the solar power system to generate electrical energy. This is an inherent limitation of most existing solar power systems which limits their applicability to generation of power on an economical basis.
From the foregoing, it will be appreciated that there is a continuing need for solar power generation systems that are capable of producing electrical power with greater efficiency. Moreover, there is an ongoing need for a system that is capable of producing electrical power during longer periods and, in particular, during time periods where the system may not be receiving direct sunlight.